This invention relates to switching-type power supplies and more particularly to a self-exciting switching D.C. stabilized power supply which utilizes feedback circuits for controlling the operating frequency of a switching transistor.
Switching D.C. stabilized power supplies are divided into self-excited and separately excited types. The latter type is usually powered with an independent oscillator section using dedicated integrated circuit elements for stabilizing the oscillating frequency but requires a separate power source for supplying voltage to the oscillation section and a choke coil on the output side of the power supply. The self-excited type of power supply is constructed to have a self-oscillating function, allowing the circuit to have a simple arrangement of parts which are fewer in number than the separately excited type, and is therefore lower in cost. Examples of this type of power supply may be found in U.S. Pat. Nos. 3,504,263 and 4,378,585.
In self-excited switching power supplies of the prior art, as the load increases, the on and off periods of the switching transistors are prolonged and consequently the oscillation period is prolonged; whereas, if the load decreases, the oscillation period is decreased. This fluctuation in the oscillation period requires high speed circuit elements which can deal with the changes in the oscillation frequency. The fluctuation in the oscillation period of the switching transistors also produces a wide band of switching noise which is also in the high frequency range. Such prior art construction of a power supply thus requires an extremely large size noise filter which increases the cost of the power supply. It is therefore a principal object of this invention to provide a self-excited switching D.C. power supply which overcomes the above-cited drawbacks of the prior art. It is a further object of this invention to provide a small size self-excited switching D.C. power supply in which the oscillating frequency of the switching transistor is kept stable, thereby restricting the switching noise to a narrow band as the load varies.